How to bond silicone with metal and plastic: primer, plasma treatment, and encapsulation

Silicone components are often not used independently: sealing rings are fitted onto metal rings, button boards are attached to PCBs, and silicone handles are wrapped around plastic skeletons. Silicone is a notoriously "difficult-to-stick material" - with low surface energy and strong chemical inertia, ordinary glues are basically ineffective on it. There is a specialized technical approach to achieve a strong bond between silicone and other materials.

Why is silicone difficult to adhere to

Silicone has a low surface energy (about 20-24 mN/m), making it impossible for most glues to wet. Its molecular chain is highly stable, with no reactive groups available for reaction. Therefore, when epoxy, instant adhesive, or hot melt adhesive is applied to silicone, it either fails to adhere or falls off easily upon removal. The "silicone glue" available in the market that can adhere to silicone relies on a special formula (de-alcoholized silicone glue, primer activation), rather than just changing the name of ordinary glue.

Three mainstream combination routes

Route 1: Mechanical rubber coating (in-mold forming)

Place the metal insert or plastic skeleton into the mold, and the silicone is directly injected/molded onto it. The bonding force comes from two aspects: physical anchoring (silicone penetrating into the grooves and through-holes on the surface of the insert) + chemical bonding (primer).

Metal insert: sandblasting for roughening + application of primer (such as Chem-Lok adhesive) + heating for curing, followed by mold encapsulation. Pure mechanical encapsulation without primer has limited bonding strength, and parts subjected to repeated stretching or torsion may delaminate.

Plastic skeleton: Please note the temperature resistance - the vulcanization temperature of silicone is 170-200℃, which most general-purpose plastics cannot withstand. The plastics that can be encapsulated are high-temperature resistant: PA (nylon), PPA, PEEK, and glass fiber-reinforced PBT. Common plastics such as ABS and PC cannot be directly encapsulated with silicone, and other methods (such as post-bonding or two-color process) must be employed.

Route 2: Plasma treatment + Post-bonding

When the molded silicone parts need to be adhered to other materials (such as being attached to a chassis), plasma treatment (or flame, primer) should be applied first to activate the surface of the silicone. With increased surface energy, ordinary double-sided tape and structural adhesive can effectively wet and bond.

Silicone seals in consumer electronics predominantly utilize the "plasma + adhesive" process: after the silicone sealing strip is die-cut, it undergoes plasma treatment, followed by the application of 3M double-sided tape. During customer assembly, the tape is removed and the seal is applied immediately. The effectiveness of plasma treatment is limited in time (from a few hours to a few days, depending on the storage environment), so the adhesive should be applied as soon as possible after treatment.

Route 3: Liquid silicone rubber injection molding on the rubber-receiving surface (LSR secondary molding/direct bonding)

Without the use of primer, the microstructure is created on the surface of the plastic part, and silicone is directly injected and adhered - the plastic part is first injection molded, and a ring groove or microporous structure is created on the surface. Then, it is placed into a silicone mold for secondary injection molding. The direct bonding process of PA+LSR is widely used in automotive connector sealing (replacing traditional O-ring assembly). This approach requires stable injection molding processes for both materials, with significant upfront development investment, and is only cost-effective for high-volume products.

How to verify the bonding strength

Peel test: 90-degree or 180-degree peeling, measuring the force in newtons per centimeter of width. Pull test: Pull in the vertical direction to measure the force that the bonding area can withstand. The form of failure is a key indicator: whether it is "adhesive interface failure" (the glue and the material separate, indicating poor bonding quality) or "material body failure" (the silicone itself tears, indicating that the bonding is stronger than the material itself - the ideal state).

Retention rate after aging: After aging in high temperature and humidity conditions or salt spray, only those with a bonding strength retention rate of over 50% can be considered reliable. Just because a sample is firmly bonded when it is first made does not mean it can be used for three years.

Common failure modes

Delamination of rubberized parts - expired primer, prolonged storage after application, insufficient vulcanization temperature leading to primer activation failure. In mass production, it is essential to manage the batch and expiration date of the primer, which serves as a process control point.

The adhesive-backed part fell off - it was left for a week after plasma treatment before applying adhesive, and the surface energy had already decayed. The process document should clearly state that "adhesive application must be completed within X hours after treatment".

No re-verification after material change - The plastic parts were sourced from a different supplier, and the material formula (with different mold release agent content) was changed, resulting in a 50% decrease in bonding strength. Material changes for secondary molding and rubber coating require re-verification for bonding, and should be documented in the change management process.

Bonding is the fundamental capability for the "1+1" application of silicone rubber parts. For products involving encapsulation and fitting, it's worth paying special attention to whether the supplier has a primer process and bonding testing methods during factory inspection - this can reveal more about the supplier's capabilities than whether the workshop is new or not.